Nozzle-area ratio float bias

ABSTRACT

A mechanism included in a convergent-divergent nozzle connected to an aft portion of a gas turbine engine, which mechanism includes a convergent flap configured to be kinematically connected to the aft portion of the gas turbine engine, a divergent flap pivotally connected to the convergent flap and configured to be kinematically connected to the aft portion of the gas turbine engine, at least one actuator configured to be connected to the aft portion of the gas turbine engine and to mechanically prescribe the position of the convergent flap and the divergent flap by moving through one or more positions, and at least one biasing apparatus. The biasing apparatus is configured to position at least one of the convergent flap and the divergent flap independent of the positions of the actuator by substantially balancing a force on the at least one of the convergent flap and the divergent flap produced by a gas pressure on the nozzle.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a divisional application from U.S. PatentApplication Ser. No. 11/894,313, filed Aug. 21, 2007.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under N00019-02-C-3003awarded by Department of the Navy. The Federal Government has certainrights in this invention.

BACKGROUND

The present invention relates to convergent-divergent nozzles used ingas turbine engines. In particular, the present invention relates toconvergent-divergent nozzles with a floating area ratio.

Prior gas turbine engines have, in some configurations, included exitnozzles attached to the aft end of the engine. Exit nozzles are commonlyemployed to produce additional thrust for the engine by accelerating theworking medium gas, for example air, leaving the aft end of the mainengine, for example via the low pressure turbine, through the nozzle.Exit nozzles accelerate the air leaving the engine, and thereforeproduce useful thrust, by prescribing the nozzle area for particularexit pressures inside the nozzle. One such exit nozzle is the variableconvergent-divergent nozzle. Prior variable convergent-divergent nozzlescommonly include convergent-divergent flap sets arrangedcircumferentially about the main axis of the engine to form asubstantially circular annular nozzle extending aft of, for example, thelow pressure turbine. The convergent-divergent flap sets are commonlyconnected to an annular ring, sometimes referred to as a sync ring,which is in turn connected to an engine casing. The convergent flap ineach of the convergent-divergent flap sets declines generally toward themain axis of the engine as the flap extends aftwardly. The divergentflap in each of the convergent-divergent flap sets may be pivotallyconnected to the convergent flap and inclines generally away from themain axis of the engine as the flap extends aftwardly. Thecircumferentially arranged convergent-divergent flap sets therefore forman annular nozzle whose cross-sectional area decreases from the forwardend of the nozzle to a throat generally defined by the pivotalconnection between the convergent and divergent flaps and then increasesfrom the throat to the nozzle exit.

In order to operate efficiently, variable convergent-divergent nozzlesare configured to position the convergent and divergent flaps, andthereby the annular shape of the entire nozzle, to optimize engineperformance. The position of the convergent and divergent flaps, andthereby the annular shape of the nozzle is commonly represented by theratio of the cross-sectional area of the nozzle at the exit (A_(E))divided by the cross-sectional area of the nozzle at the throat (A_(T)),or A_(E)/A_(T). The nozzle pressure ratio (NPR) is equal to the totalpressure at the nozzle throat (P_(T)) divided by the ambient pressure(P_(Amb)), or NPR=P_(T)/P_(Amb). Convergent-divergent nozzles functiongenerally by designing A_(E)/A_(T) for critical flight conditions (NPR)in order to produce useful thrust by extracting as much energy as ispracticable from the working medium gas flowing through the nozzle.

Prior variable convergent-divergent nozzles have used various means tovary the position of the convergent and divergent flaps for differentengine conditions. For example, some prior convergent-divergent nozzleshave mechanically prescribed the position of the convergent anddivergent flaps through a kinematic mechanism driven by one or moreactuators to tune A_(E)/A_(T) for specific engine conditions. Priorconvergent-divergent nozzles have also employed kinematics that varyA_(E)/A_(T) with respect to A_(T) to achieve improved performance atmultiple engine operating conditions. This arrangement allows for asingle valued A_(E)/A_(T) for all A_(T) without the weight andcomplexity of independently controlling A_(E)/A_(T). Other priorconvergent-divergent nozzles have employed a toggling configurationtriggered by the pressure inside the nozzle, which acts to position thedivergent flaps for low and high A_(E) respectively (low and high mode).Nozzles employing such a toggling configuration are considered to havetwo available values of A_(E) for each A_(T). At low A_(T), which istypical of aircraft cruise and low values of NPR, a low value ofA_(E)/A_(T) is desirable. At relatively high values of A_(T) a highervalue of A_(E)/A_(T) is desirable, which corresponds to conditionsassociated with aircraft acceleration. Thus the low mode (lowA_(E)/A_(T)) condition corresponds to relatively low values of NPR andhigh mode (higher A_(E)/A_(T)) corresponds to relatively high values ofNPR.

Prior variable convergent-divergent nozzles have several disadvantageswith respect to A_(E)/A_(T). In prior nozzles independently controllingA_(T) and A_(E), one disadvantage is weight and complexity (design andcontrol). For scheduled (single valued) A_(E)/A_(T) nozzles, onedisadvantage is an inability to run optimally at low and high NPR(cruise and acceleration). More generally, prior nozzle designs havevaried the nozzle geometry to optimally position the convergent anddivergent flaps at low mode and high mode, but have failed to vary theconvergent and divergent flaps position through an intermediate mode ofengine operation between low and high modes. Therefore, prior nozzleconfigurations have failed to advantageously position the convergent anddivergent flaps for a substantial number of NPR values encounteredduring engine operation, thereby causing sub-optimal engine performanceat many of the NPR values encountered.

SUMMARY

Embodiments of the present invention include a mechanism included in aconvergent-divergent nozzle connected to an aft portion of a gas turbineengine, which mechanism includes a convergent flap configured to bekinematically connected to the aft portion of the gas turbine engine, adivergent flap pivotally connected to the convergent flap and configuredto be kinematically connected to the aft portion of the gas turbineengine, at least one actuator configured to be connected to the aftportion of the gas turbine engine and to mechanically prescribe theposition of the convergent flap and the divergent flap by moving throughone or more positions, and at least one biasing apparatus. The biasingapparatus is configured to position at least one of the convergent flapand the divergent flap independent of the positions of the actuator bysubstantially balancing a force on the at least one of the convergentflap and the divergent flap produced by a gas pressure on the nozzle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial section view of a gas turbine engine including avariable convergent-divergent nozzle according to the present invention.

FIG. 2 is a side view of one mechanism included in theconvergent-divergent nozzle of FIG. 1.

FIGS. 3 and 4 show graphs of nozzle area ratio as a function of NPR,which illustrate performance of nozzles according to the presentinvention.

FIGS. 5 and 6 are side views of alternative embodiments of mechanismsincluded in convergent-divergent nozzles according to the presentinvention.

DETAILED DESCRIPTION

FIG. 1 is an axial section view of gas turbine engine 10 includingengine axis 12, fan 14, compressor 16, combustion section 18, turbine20, and variable convergent-divergent nozzle 22. Nozzle 22 includes aplurality of mechanisms 24, which mechanisms 24 include a plurality ofconvergent flaps 26 and divergent flaps 28 circumferentially arrangedaround engine axis 12. During operation of engine 10, working medium gasstream 30, for example a stream of air, is pulled into the front ofengine 10 by fan 14, by the rotation of fan blades about axis 12. Fan 12directs a portion of gas stream 30 into compressor 16. Working mediumgas 30 is successively compressed through stages of compressor 16 anddirected into combustor section 18. In combustor section 18, gas stream30 is mixed with fuel and ignited. The gas and fuel mixture ignited incombustor section 18 is directed into turbine 20 in which the mixture issuccessively expanded. A portion of the energy of the gas and fuelmixture expanded through turbine 20 powers fan 14 and compressor 16through co-joined shafting. The remaining portion of the energy of thegas and fuel mixture expanded through turbine 20 exits the back ofengine 10 through nozzle 22 to produce useful thrust for engine 10.

Nozzle 22 augments the thrust produced by the gas and fuel mixtureexpanded through turbine 20 by accelerating the gas and fuel mixturethrough the exit of engine 10. Nozzle 22 accelerates the gas and fuelmixture leaving the engine, and thereby produces additional thrust, byprescribing the nozzle area for particular exit pressures inside thenozzle. Specifically, nozzle 22 in FIG. 1 includes convergent anddivergent flaps 26, 28 circumferentially arranged to form a generallycircular annular nozzle 22. The aft end of convergent flaps 26 arepivotally connected to the forward end of divergent flaps 28. Thecross-sectional area of nozzle 22 decreases from the forward end ofconvergent flaps 26 to nozzle throat 32 defined by a plane perpendicularto engine axis 12 and passing through the pivotal connection betweenconvergent and divergent flaps 26, 28. From nozzle throat 32, thecross-sectional area of nozzle 22 increases toward engine exit 34.Nozzle 22 accelerates the gas and fuel mixture leaving turbine 20 byprescribing the ratio of the area of nozzle 22 at engine exit 34 (A_(E))divided by the area of nozzle 22 at nozzle throat 32 (A_(T)) fordifferent pressures inside nozzle 22. As discussed above, nozzlepressures are generally represented by NPR, which, in FIG. 1, is equalto the pressure at nozzle throat 32 (P_(T)) divided by the ambientpressure (P_(Amb)), or NPR=P_(T)/P_(Amb). Therefore, nozzle 22prescribes A_(E)/A_(T) by positioning convergent and divergent flaps 26,28, as a function of varying values of NPR encountered during operationof engine 10.

FIG. 2 is a side view of mechanism 24 included in nozzle 22 of FIG. 1.Mechanism 24 includes convergent flap 26, divergent flap 28, actuator36, first link 38, second link 40, third link 42, fourth link 44, anddivergent flap rotation center 46. In FIG. 2, the forward end ofconvergent flap 26 is pivotally connected to the aft end of engine 10(shown in FIG. 1). In alternative embodiments of mechanism 24, theforward end of convergent flap 26 may be pivotally attached to anintermediate link attached to the aft end of engine 10, or convergentflap 26 may be pivotally attached to an actuator attached to the aft endof engine 10. The aft end of convergent flap 26 is pivotally connectedto the forward end of divergent flap 28 at which connection generallydefines nozzle throat 32. Actuator 36 is movably connected to the aftend of engine 10. First link 38 is pivotally connected between actuator36 and second link 40. Second link 40 is pivotally connected betweenfirst link 38, the aft end of engine 10, and third link 42. Third link42 is pivotally connected between second link 40 and divergent flap 28.Fourth link 44 is connected between the aft end of engine 10 anddivergent flap 28 toward a middle portion of divergent flap 28. Fourthlink 44 may be, as shown in FIG. 2, pivotally and slidably connected todivergent flap 28.

Mechanism 24 is configured to mechanically and aerodynamically prescribethe position of convergent and divergent flaps 26, 28 as a function ofNPR. Specifically, mechanism 24 is a kinematic mechanism including theinterconnection of convergent flap 26, divergent flap 28, and first,second, third, and fourth links 38-44 and mechanically driven byactuator 36. Actuator 36 is configured to move forward and aftwardthrough one or more positions, which actuator 36 movement mechanicallydrives mechanism 24 and thereby positions convergent and divergent flaps26, 28. The movement of actuator 36 may be controlled to positionconvergent and divergent flaps 26, 28 for varying values of NPRencountered during operation of engine 10.

Independent of the movement of actuator 36, mechanism 24 is configuredto vary the position of convergent and divergent flaps 26, 28 as afunction of the gas pressure acting on nozzle 22 during operation ofengine 10. Although mechanism 24 is configured to aerodynamically varyconvergent and divergent flaps 26, 28 as a function of NPR, embodimentsof the present invention also include configurations capable ofaerodynamically prescribing the position of the convergent flap 26 butnot the divergent flap 28 and configurations capable of aerodynamicallyprescribing the position of the divergent flap 28 but not the convergentflap 26. In FIG. 2, fourth link 44 is pivotally and slidably connectedto divergent flap 28. The sliding connection between fourth link 44 anddivergent flap 28 is configured to impart a degree of freedom ofmovement to divergent flap 28 independent of the mechanical positioningof mechanism 24 by actuator 36. For example, in FIG. 2 the slidingconnection between fourth link 44 and divergent flap 28 is at a radiallyoutward limit. However, as the pressure acting on nozzle 22 changes,divergent flap 28 may react to a force created by the pressure acting onnozzle 22 by rotating counter-clockwise about divergent flap rotationcenter 46 and moving along the sliding connection with link 44. In thisexample, the movement of divergent flap 28 is configured with amechanical limit prescribed by the radially inward limit of the slidingconnection with link 44.

In order to prescribe one or more positions of convergent and divergentflaps 26, 28 as a function of NPR, as opposed to only imparting anadditional degree of freedom of movement, mechanism 24 includes biasingapparatus 48 configured to substantially balance the pressure acting torotate convergent and divergent flaps 26, 28. In the embodiment of FIG.2, biasing apparatus 48 is an offset hinge created by offsettingdivergent flap rotation center 46 from the pivotal connections betweendivergent flap 28 and convergent flap 26 and third link 42. Biasingapparatus 48 is configured to employ the pressure inside nozzle 22acting on convergent flap 26 to counteract and substantially balance thepressure inside nozzle 22 acting on divergent flap 28.

In operation at a given NPR value, mechanism 24 may mechanicallyprescribe the position of convergent and divergent flaps 26, 28 by themovement of actuator 36 and the kinematic interconnections of convergentand divergent flaps 26, 28, and first, second, third, and fourth links38-44. Once the position of convergent and divergent flaps 26, 28 ismechanically prescribed, convergent and divergent flaps 26, 28 may floatthrough a range of positions based on changing NPR values as thepressure inside nozzle 22 changes and biasing apparatus 48 automaticallyadjusts to substantially balance the pressure at a new convergent anddivergent flap 26, 28 position. Mechanism 24 is configured tomechanically limit the range of floating positions of convergent anddivergent flap 26, 28 achieved by the interaction between the pressureinside nozzle 22 and the balancing action of biasing apparatus 48.During engine operation, for example, at and below an NPR low modevalue, the position of convergent and divergent flaps 26, 28 isconfigured to remain substantially fixed and is mechanically prescribedby the position of actuator 36. Between the NPR low mode value and anNPR high mode value, convergent and divergent flaps 26, 28 areconfigured to float through a range of positions as the pressure insidenozzle 22 is substantially balanced by biasing apparatus 48. At andabove the NPR high mode value, the position of convergent and divergentflaps 26, 28 is configured to remain substantially fixed and ismechanically prescribed by the position of actuator 36. In theembodiment of FIG. 2, the mechanical limit on the float range ofconvergent and divergent flaps 26, 28 is provided by fourth link 44 andthe pivotal and sliding connection between fourth link 44 and divergentflap 28.

Therefore, in FIG. 2, mechanism 24 may be configured to mechanicallyproscribe the position of convergent flaps 26, 28 at and below NPR lowmode and at and above NPR high mode. Link 44 may be adjusted fordifferent operating conditions of engine 10 to provide a mechanicallimit on the float range of convergent and divergent flaps 26, 28 andthereby mechanically prescribe a particular convergent and divergentflap 26, 28 position at particular NPR low and high mode values.Mechanism 24, independent of mechanically proscribing the position ofconvergent and divergent flaps 26, 28, also may be configured to floatthe position of convergent and divergent flaps 26, 28 through a range ofpositions as the pressure inside nozzle 22 at values of NPR between highand low modes is substantially balanced by biasing apparatus 48. Theoffset hinge comprising biasing apparatus 48 may be adjusted to adaptmechanism 24 to different operating conditions of engine 10, i.e. tosubstantially balance different pressures inside nozzle 22.

FIGS. 3 and 4 illustrate performance of nozzles according to the presentinvention by showing graphs of nozzle area ratio (A_(E)/A_(T)) as afunction of NPR (P_(T)/P_(Amb)). FIG. 3 shows a graph of A_(E)/A_(T) asa function of NPR for one embodiment of a nozzle according to thepresent invention including a plurality of mechanisms configuredsimilarly to mechanism 24 shown in FIGS. 1 and 2. FIG. 4 shows a graphof A_(E)/A_(T) as a function of NPR for two embodiments of nozzlesaccording to the present invention under different operating conditionsand for a common prior convergent-divergent nozzle configuration.

In FIG. 3, A_(E)/A_(T) is plotted as a function of NPR for a nozzleincluding a plurality of mechanisms configured similarly to mechanism24. FIG. 3 generally shows NPR values ranging from low mode 52, to floatmode 54, to high mode 56 and includes first performance curve 58 andsecond performance curve 60. First performance curve 58 represents idealA_(E)/A_(T), and thereby ideal nozzle geometry variation, for NPR valuesranging from low mode 52 to high mode 56. Second performance curve 60shows A_(E)/A_(T) for one embodiment of a nozzle according to thepresent invention for NPR values ranging from low mode 52 to high mode56. Nozzles according to the present invention are adapted to floatthrough a range of A_(E)/A_(T) values between low mode 52 and high mode56, as illustrated by the non-linear portion of curve 60 in FIG. 3.Nozzles according to the present invention may be configured fordifferent engine operating conditions to achieve nozzle performance infloat mode 54 approximating ideal performance (illustrated in FIG. 3 assecond performance curve 60). Performance adjustments in float mode 54may be achieved by adapting biasing apparatuses included in the nozzleto engine operating conditions. For example, nozzle performance may beadjusted by adapting the biasing force to substantially balance pressureconditions, and thereby define A_(E)/A_(T) by positioning the convergentand divergent flaps of the nozzle, through a range of NPR values betweenlow mode 52 and high mode 56.

FIG. 4 shows a graph of A_(E)/A_(T) as a function of NPR for twoembodiments of nozzles according to the present invention underdifferent operating conditions and for a common priorconvergent-divergent nozzle configuration. The graph of FIG. 4 includesthird performance curve 62, fourth performance curve 64, and fifthperformance curve 66. Third and fourth performance curves 62, 64represent the performance of two embodiments of nozzles according to thepresent invention adapted to different engine operating conditions.Third performance curve 62 includes NPR low mode 62 a, NPR float mode 62b, and NPR high mode 62 c. Fourth performance curve 64 includes NPR lowmode 64 a, NPR float mode 64 b, and NPR high mode 64 c.

FIG. 4 illustrates three advantages of convergent-divergent nozzlesaccording to the present invention over prior nozzle configurations.First, nozzles according to the present invention are adapted to includea float mode, for example NPR float mode 62 b or NPR float mode 64 b. Asillustrated in FIG. 4, prior nozzle configurations are substantiallyincapable of floating, i.e. varying A_(E)/A_(T) as a function of NPR,between low mode and high mode. Rather, prior nozzle configurationsmechanically or aerodynamically toggle the position of convergent anddivergent flaps to achieve A_(E)/A_(T) between low and high mode, whichis represented in FIG. 4 by fifth performance curve 66 stepping from alow mode A_(E)/A_(T) to a high mode A_(E)/A_(T) at a given NPR value.Second, the range of A_(E)/A_(T) over which nozzles according to thepresent invention transition from low mode to high mode is substantiallygreater than in prior nozzles, which is illustrated by the height of thestep in fifth performance curve 66 versus the height of NPR float mode62 b in third performance curve 62 and the height of NPR float mode 64 bin fourth performance curve 64. Third, nozzles according to the presentinvention may adapt to different engine NPR modal shifts by adjustingthe NPR values at which A_(E)/A_(T) shifts from low mode to float modeto high mode. For example, in mechanism 24 of FIGS. 1 and 2, by varyingthe configuration of fourth link 44 and the pivotal and slidingconnection between fourth link 44 and divergent flap 28 the modal shiftsof mechanism 24 may be adapted to different engine configurations andoperating conditions. In FIG. 4, this performance adjustment isillustrated by the horizontal offset between third performance curve 62including NPR low, float, and high modes 62 a, 62 b, 62 c and fourthperformance curve 64 including NPR low, float, and high modes 64 a, 64b, 64 c.

FIGS. 5 and 6 are side views of alternative embodiments of mechanismsincluded in convergent-divergent nozzles according to the presentinvention. FIG. 5 is a side view of mechanism 24 including alternativebiasing apparatus 68. FIG. 6 is a side view of mechanism 24 includingalternative biasing apparatus 72. In FIG. 5, mechanism 24 includesbiasing apparatus 68 located at the pivotal and sliding connectionbetween fourth link 44 and divergent flap 28. Biasing apparatus 68 maybe employed in lieu of or, as shown in FIG. 5, in addition to biasingapparatus 48. Biasing apparatus 68 may include one or more biasingmembers, for example, one or more compression springs 70 as shown inFIG. 5. In FIG. 6, mechanism 24 includes biasing apparatus 72 located atthe connection between fourth link 44 and the aft end of engine 10.Biasing apparatus 72 may be employed in lieu of or in addition tobiasing apparatus 48 or biasing apparatus 68 shown in FIG. 5. Biasingapparatus 72 may include one or more biasing members, for example,solenoid 74 as shown in FIG. 6. Biasing apparatuses 68 and 72, either incombination with each other or with biasing apparatus 48 or bythemselves, may be configured to position divergent flap 28 bysubstantially balancing a force produced by a gas pressure acting ondivergent flap 28. Embodiments of the present invention also includeother alternative biasing apparatus arrangements not shown in FIGS. 2,5, and 6. For example, mechanism 24 may include a biasing apparatusincluding one or more torsional springs located at the pivotalconnection between link 44 and the aft end of engine 10. In anotherembodiment, link 44 may act as a biasing apparatus, for example, byincluding a fluid pressure biasing member such as a pneumatic orhydraulic cylinder.

Convergent-divergent nozzles and gas turbine engines includingconvergent-divergent nozzles according to the present invention haveseveral advantages over prior nozzle configurations. Nozzles accordingto the present invention include a plurality of mechanismscircumferentially arranged about a main axis of the gas turbine engineand configured to mechanically and aerodynamically position convergentand divergent flaps as a function of a gas pressure on the nozzle. Eachof the plurality of mechanisms include at least one biasing apparatusconfigured to position at least one of the convergent and divergentflaps independent of a mechanically prescribed position by substantiallybalancing a force produced by a gas pressure acting on the nozzle.During engine operation, nozzles according to the present invention areconfigured to provide a float mode between NPR low and high modes inwhich the position of one or both the convergent and divergent flapsfloat through a range of positions as a function of the changingpressure on the nozzle. Also, nozzles according to the present inventionare configured to provide a wider range of convergent and divergent flappositions, and thereby nozzle geometry (A_(E)/A_(T)), between NPR lowand high modes than prior nozzles. Finally, nozzles according to thepresent invention may be adjusted for varying NPR low to high modalshifts, thereby adjusting nozzle performance for different engineconfigurations and operating conditions. Nozzles according to thepresent invention therefore substantially increase engine efficiency andperformance over prior nozzle configurations by advantageouslypositioning the convergent and divergent flaps for a substantiallygreater number of NPR values encountered during engine operation.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

1. A convergent-divergent nozzle of a gas turbine engine comprising: anannular ring configured to be connected to an aft portion of the gasturbine engine; a plurality of mechanisms circumferentially connected tothe annular ring about a main axis of the gas turbine engine; whereinthe plurality of mechanisms each comprise: a convergent flap configuredto be kinematically connected to the aft portion of the gas turbineengine; a divergent flap pivotally and directly connected to theconvergent flap at a hinged connection; at least one actuator connectedto the annular ring and configured to mechanically prescribe theposition of the convergent flap and the divergent flap by moving throughone or more positions; and at least one biasing apparatus configured toposition the divergent flap independent of the one or more positions ofthe actuator by substantially balancing a force on the divergent flapproduced by a gas pressure on the nozzle, the at least one biasingapparatus comprising: a biasing linkage directly connected to thedivergent flap through a pivotal and sliding connection, the biasinglinkage configured to kinematically connect the divergent flap to theaft portion of the gas turbine engine; and a spring connected to thebiasing linkage and the annular ring.
 2. The nozzle of claim 1, whereinthe hinged connection comprises an offset hinge between the convergentflap and the divergent flap configured to provide a center of rotationof the divergent flap offset from the hinged connection between thedivergent flap and the convergent flap.
 3. The nozzle of claim 2,wherein the offset hinge is configured to position the divergent flap bysubstantially balancing a force produced by a gas pressure on thedivergent flap with a force produced by a gas pressure on the convergentflap.
 4. The nozzle of claim 2, wherein the offset hinge comprises oneor more links configured to be kinematically connected between the aftportion of the gas turbine engine and the divergent flap toward thehinged connection between the divergent flap and the convergent flap. 5.The mechanism of claim 4, wherein the one or more links comprise: afirst link pivotally connected to the at least one actuator; a secondlink pivotally connected to the first link and configured to bepivotally connected to the aft portion of the gas turbine engine; and athird link pivotally connected to the second link and the divergent flaptoward the hinged connection between the divergent flap and theconvergent flap.
 6. The nozzle of claim 1, wherein the biasing linkageis configured to be pivotally connected to the aft portion of the gasturbine engine at a pivotal connection and pivotally and slidablyconnected to the divergent flap toward a middle portion of the divergentflap.
 7. The nozzle of claim 6, wherein the at least one biasingapparatus comprises one or more biasing members in addition to thespring, wherein the one or more biasing members and the spring areconfigured to bias the pivotal and sliding connection between thedivergent flap and the biasing linkage and the pivotal connectionbetween the biasing linkage and the aft portion of the gas turbineengine.
 8. The nozzle of claim 7, wherein the one or more biasingmembers are selected from a group of biasing members comprising springs,fluid pressurized members, and mechanical and electro-mechanicalactuators.
 9. A variable exhaust nozzle system for a gas turbine engine,the variable exhaust nozzle comprising: an annular ring configured to beconnected to an aft end of a gas turbine engine; a plurality ofmechanisms circumferentially connected to the annular ring about a mainaxis of the gas turbine engine, each of the plurality of mechanismscomprising: a convergent flap having: a first end pivotally connected tothe annular ring; and a second end; a divergent flap having: a first endpivotally connected to the second end of the convergent flap at a hingedconnection; and a second end; an actuator mounted to the annular ringand configured to mechanically prescribe the position of the convergentflap and the divergent flap; a biasing linkage having: a first endproximate the annular ring; and a second end connected to the divergentflap at a pivotal and sliding connection; and a solenoid connected tothe biasing linkage to position the divergent flap independent of theactuator.
 10. The variable exhaust nozzle system of claim 9 wherein thehinged connection comprises an offset hinge between the convergent flapand the divergent flap to provide a center of rotation of the divergentflap offset from the hinged connection between the divergent flap andthe convergent flap.
 11. The variable exhaust nozzle system of claim 10wherein the offset hinge comprises: a first link pivotally connected tothe actuator; a second link pivotally connected to the first link andconfigured to be pivotally connected to the aft portion of the gasturbine engine; and a third link pivotally connected to the second linkand the divergent flap toward the hinged connection between thedivergent flap and the convergent flap.
 12. The variable exhaust nozzlesystem of claim 9 wherein the solenoid is connected to the second end ofthe biasing linkage and the diverging flap.
 13. The variable exhaustnozzle system of claim 9 wherein the solenoid is connected to the firstend of the biasing linkage and the annular ring.
 14. The variableexhaust nozzle system of claim 9 and further comprising a springconnected to the biasing linkage and configured to bias the pivotal andsliding connection between the divergent flap and the biasing linkage.15. A variable exhaust nozzle system for a gas turbine engine, thevariable exhaust nozzle comprising: an annular ring configured to beconnected to an aft end of a gas turbine engine; a plurality ofmechanisms circumferentially connected to the annular ring about a mainaxis of the gas turbine engine, each of the plurality of mechanismscomprising: a convergent flap having: a first end pivotally connected tothe annular ring; and a second end; a divergent flap having: a first endpivotally connected to the second end of the convergent flap; and asecond end; an actuator mounted to the annular ring and configured tomechanically prescribe the position of the convergent flap and thedivergent flap; a biasing linkage having: a first end proximate theannular ring; and a second end connected to the divergent flap at ajoint allowing rotational freedom and slidable freedom; and a firstbiasing member connected to the biasing linkage to restrict the slidablefreedom of the joint.
 16. The variable exhaust nozzle system of claim 15wherein the first biasing member is further connected to the annularring.
 17. The variable exhaust nozzle system of claim 15 wherein thefirst biasing member comprises a spring.
 18. The variable exhaust nozzlesystem of claim 15 wherein the first biasing member comprises asolenoid.
 19. The variable exhaust nozzle system of claim 15 wherein thefirst biasing member comprises a fluid pressurized member.
 20. Thevariable exhaust nozzle system of claim 15 and further comprising asecond biasing member connected to the biasing linkage, the secondbiasing member selected from the group consisting of a spring, a fluidpressurized member, a mechanical actuator and an electro-mechanicalactuator.